fileops: fix creation of directory in filesystem root

In commit 45f24e78 (git_repository_init: stop traversing at
windows root, 2019-04-12), we have fixed `git_futils_mkdir` to
correctly handle the case where we create a directory in
Windows-style filesystem roots like "C:\repo".

The problem here is an off-by-one: previously, to that commit,
we've been checking wether the parent directory's length is equal
to the root directory's length incremented by one. When we call
the function with "/example", then the parent directory's length
("/") is 1, but the root directory offset is 0 as the path is
directly rooted without a drive prefix. This resulted in `1 == 0 +
1`, which was true. With the change, we've stopped incrementing
the root directory length, and thus now compare `1 <= 0`, which
is false.

The previous way of doing it was kind of finicky any non-obvious,
which is also why the error was introduced. So instead of just
re-adding the increment, let's explicitly add a condition that
aborts finding the parent if the current parent path is "/".

Making this change causes Azure Pipelines to fail the testcase
repo::init::nonexistent_paths on Unix-based systems. This is because we
have just fixed creating directories in the filesystem root, which
previously didn't work. As Docker-based tests are running as root user,
we are thus able to create the non-existing path and will now succeed to
create the repository that was expected to actually fail.

Let's split this up into three different tests:

- A test to verify that we do not create repos in a non-existing parent
  directoy if the flag `GIT_REPOSITORY_INIT_MKPATH` is not set.

- A test to verify that we fail if the root directory does not exist. As
  there is a common root directory on Unix-based systems that always
  exist, we can only test for this on Windows-based systems.

- A test to verify that we fail if trying to create a repository in an
  unwriteable parent directory. We can only test this if not running
  tests as root user, as CAP_DAC_OVERRIDE will cause us to ignore
  permissions when creating files.

src/fileops.c

@@ -495,7 +495,9 @@ int git_futils_mkdir(
 		 * equal to length of the root path).  The path may be less than the
 		 * root path length on Windows, where `C:` == `C:/`.
 		 */
-		if ((len == 1 && parent_path.ptr[0] == '.') || len <= root_len) {
+		if ((len == 1 && parent_path.ptr[0] == '.') ||
+		    (len == 1 && parent_path.ptr[0] == '/') ||
+		    len <= root_len) {
 			relative = make_path.ptr;
 			break;
 		}

tests/repo/init.c

@@ -878,14 +878,55 @@ void test_repo_init__at_filesystem_root(void)
 	git_repository_free(repo);
 }
 
-void test_repo_init__nonexistent_paths(void)
+void test_repo_init__nonexisting_directory(void)
 {
+	git_repository_init_options opts = GIT_REPOSITORY_INIT_OPTIONS_INIT;
 	git_repository *repo;
 
+	/*
+	 * If creating a repo with non-existing parent directories, then libgit2
+	 * will by default create the complete directory hierarchy if using
+	 * `git_repository_init`. Thus, let's use the extended version and not
+	 * set the `GIT_REPOSITORY_INIT_MKPATH` flag.
+	 */
+	cl_git_fail(git_repository_init_ext(&repo, "nonexisting/path", &opts));
+}
+
+void test_repo_init__nonexisting_root(void)
+{
 #ifdef GIT_WIN32
+	git_repository *repo;
+
+	/*
+	 * This really only depends on the nonexistence of the Q: drive. We
+	 * cannot implement the equivalent test on Unix systems, as there is
+	 * fundamentally no path that is disconnected from the root directory.
+	 */
 	cl_git_fail(git_repository_init(&repo, "Q:/non/existent/path", 0));
 	cl_git_fail(git_repository_init(&repo, "Q:\\non\\existent\\path", 0));
 #else
-	cl_git_fail(git_repository_init(&repo, "/non/existent/path", 0));
+	clar__skip();
+#endif
+}
+
+void test_repo_init__unwriteable_directory(void)
+{
+#ifndef GIT_WIN32
+	git_repository *repo;
+
+	if (geteuid() == 0)
+		clar__skip();
+
+	/*
+	 * Create a non-writeable directory so that we cannot create directories
+	 * inside of it. The root user has CAP_DAC_OVERRIDE, so he doesn't care
+	 * for the directory permissions and thus we need to skip the test if
+	 * run as root user.
+	 */
+	cl_must_pass(p_mkdir("unwriteable", 0444));
+	cl_git_fail(git_repository_init(&repo, "unwriteable/repo", 0));
+	cl_must_pass(p_rmdir("unwriteable"));
+#else
+	clar__skip();
 #endif
 }